Reaction pathways on NiMo/Al2O3 catalysts for hydrodesulfurization of diesel fuel

The ranking of catalytic activities of several NiMo catalysts was performed using various model compounds: thiophene (TH), tetrahydrothiophene (ThTH), dibenzothiophene (DBT) and 4,6-dimethyidibenzothiophene (4,6-DMDBT). So-called type I and type II NiMo catalysts were prepared by varying Mo loading from below to above monolayer coverage. Type I and II differ most at high loading. Below monolayer surface coverage, type II active phase shows higher HG activity than type I. It is likely that type II active phase consists of higher stacking active phases, favoring the HG pathway. Consequently, type II catalyst shows higher HDS activity for TH and 4,6-DMDBT, as these sulfur compounds react mainly via HG pathway. Above monolayer surface coverage, type I active phase forms aggregated crystals on the support alumina, resulting in a low dispersion, whereas type II active phase can maintain high dispersion. Higher loading type I catalyst shows lower DDS activity and only slightly increased HG activity. Higher loading type II phase shows higher DDS and extremely high hydrogenation (HG) activity and, as a consequence, high HDS activity for TH and 4,6-DMDBT. In order to be able to generalize the results, we included a CoMo catalyst in the study. The results at first sight are not fully consistent. Although this catalyst shows a low HG activity in HDS of DBT, it shows a relatively high activity in HDS of 4,6-DMDBT. Two reaction schemes were examined, one of them taking into account a dehydrogenation step. Although a dehydrogenation step could occur under our experimental conditions, the main cause of ranking differences is the differences of susceptibility for NiMo and CoMo catalysts to H2S. (c) 2005 Elsevier B.V. All rights reserved.